Seawater Pump Corrosion Protection

ABSTRACT

Assembly for the cathodic protection of a from galvanic corrosion suffering active component, which transports corroding medium, within a steel tube or casing containing a corroding medium. A plurality of anodes is equally distributed across the annular gap, while avoiding that these anodes are galvanically contacting the active component, e.g. pump, and one or more of the casing and tube or other component within the casing, other then through the corroding seawater, and a switching galvanic circuit. These anodes are suspended from a riser contained within the casing and providing the extension of the active component.

INTRODUCTION

This invention relates to application of sacrificial anodes for the cathodic protection of an active component, such as a seawater lift pump or different active component which transports corroding medium, within a steel tube (so called casing or caisson) containing a corroding medium. E.g. an assembly for the cathodic protection of a from galvanic corrosion suffering active component, which transports corroding medium, within a steel tube containing a corroding medium within the annular gap with the active component.

Generally, the sacrificial anode consists of a zinc, aluminium or magnesium alloy. The metal part to be protected is galvanically more noble than the sacrificial anode. The sacrificial anode is present within the casing such that it is flooded by the corroding medium and can thus do its job. For more about cathodic protection (hereafter named CP), refer to guideline NORSOK RP B401 as an example.

For protection against damages due to mechanical loads, e.g. from sea waves, delicate useful components suspended from a marine structure are typically mounted in a circumferentially spaced steel protective element or shell, typically a casing or caisson or such tube like part. Examples of such delicate components are transport tubes, valves and pumps. It is known that galvanic corrosion often develops between these concentric located parts.

PRIOR ART SOLUTION

It is already known to mount sacrificial anodes in the gap or annular space between the steel (typically carbonsteel) casing flooded with seawater and a steel (typically stainless steel, thus galvanically more noble compared to the carbonsteel casing) seawater transport tube (an inactive component) concentrically located within said steel casing. Said mounted sacrificial anodes extend longitudinally relative to the steel tube and a number of such anodes of equal length extend next to each other and are with equal spacing, keeping a mutual gap, located around the steel tube with their mutually registered longitudinal ends fixedly mounted to the steel tube while being spaced from the steel tube and steel casing for their complete length between said longitudinal ends. For the mounting at each longitudinal end of the assembly of anodes a metal bracelet is used which is tightly fitted around the seawater transport tube. Thus the anodes are merely through the bracelets galvanically connected to the seawater transport tube. At a remote location, typically the top longitudinal end of the steel casing, the casing and seawater transport tube are galvanically connected.

It has come out that this way of mounting of anodes around an active component, such as a water pump, within a casing does not provide the desired protection against galvanic corrosion. Besides, the water intake is obstructed by this known assembly of anodes in case the pump intake is present within the annular gap between the casing and pump, which typically is the case. The prior art solves this problem by application of a protective coating onto one or both mutually facing surfaces of the active component and the casing. However, practise has learned that this provides only limited protection against galvanic corrosion.

OBJECT OF THE INVENTION

The object of the present invention is to offer improved protection against galvanic corrosion while obstructing the proper functioning of the active component is avoided.

The term ‘active’ here means a component with one or more moving parts, which are preferably internally located, which parts are preferably moved in a positive sense, e.g. because they are motor driven or differently actuated in a positive sense.

DEFINITION OF THE INVENTION

According to the invention anodes of substantially equal length are suspended from only a single longitudinal end and are circumferentially distributed around the active component while extending next to each other (co-extending) and being spaced from both the casing and active component, while remote from their suspended end the anodes are interconnected by a circumferentially extending spacing bracelet, providing a closed loop around and spaced from both the casing and active component and galvanic isolated spacers are active between the assembly of anodes, on the one hand, and the casing and/or active component, on the other hand, to keep the spacing bracelet, and thus the associated anodes, in a predetermined, preferably centered position relative to the casing and active component.

Preferably, one or more of the following applies: the active component is preferably concentrically mounted within a circumferentially spaced metal, e.g. steel, protective element or shell, typically a casing or caisson or such tube like part; the active component is a delicate component such as a valves or pump or drive motor; the gap or annular space is flooded with seawater; the gap has a width at least 5 or 10 centimetre and/or not more then 30 or 50 centimetre or 1 (one) or 1.50 (one and a half) metre; the diameter of the casing is at least 50 centimetre or 1 (one) metre and/or not more then 1 (one) or 1.50 (one and a half) or 2 (two) or 5 (five) metre; the diameter of the water transport tube or lift pump or different active component is at least 10 or 20 centimetre and/or not more then 75 centimetre or 1 (one) or 1.50 (one and a half) metre; at least two or three axially spaced spacing bracelets are applied; spacing bracelets are only located at the longitudinal ends of the anodes; a spacing bracelet is present between two axially spaced anodes; the spacers are mounted to the spacing bracelet; the active component is galvanically more noble relative to the casing, e.g. is made from bronze or stainless steel while the casing is made from carbon steel, preferably substantially for its complete length; the casing is substantially continuous along its complete length; the active component is an extension of another component wherein said two components are galvanically mutually connected; the anodes are with their top end fitted to the water transport tube above the seawater lift pump, while their bottom end is at the level of the seawater lift pump or its drive; the back of the anodes face the casing; one side of each anode (the back) is completely covered by a backing substrate or coating, preferably providing the means for supporting and mounting the anodes (so called flush anodes); the flush anodes are facing towards or away from the more noble component (e.g. of stainless steel or bronze, such as the pump), preferably at the one location, such as immediately opposite the tube they face away from the more noble component (e.g. the water transport tube within the casing) while at a different location, axially remote from the one location, they face (facing side means anode side opposite coated anode side) towards a different more noble component (e.g. the pump); a feature changing the galvanic current density locally (and thus the galvanic protection potential) by changing the galvanic resistance of the anodes, e.g. by having an anode at the one location facing in the one radial direction and a different anode at another, axial remote location facing in the opposite radial direction, or using at different axial locations anodes with different geometry and/or dimensions; the anodes are elongated; the anodes cover an axial length of at least 1 metre; the anodes are merely mounted to the tube or other component within the casing such that said tube/component together with the anodes can be readily extracted axially from the casing; the tube or other component within the casing is mounted to the casing at only a single axial location; the anodes and the component(s) they protect are mounted such that they exclusively make galvanic contact with each other through the corroding medium and a galvanic circuit, such as an active element or switching element, wherein the anode is preferably used as the only/exclusive energy source such that the galvanic circuit can supply a pulsating current and/or voltage and/or automatic adapted galvanic current and/or protection potential (as is known as such from WO2007 126308, Heselmans J. J. M.).

Preferably a plurality of anodes is distributed, preferably equally, across the annular gap, circumferentially and/or axially, while avoiding that these anodes are galvanically contacting the active component, e.g. pump, and one or more of the casing and tube or other component within the casing, other then through the corroding medium, e.g. sea water, and the, preferably active or switching, galvanic circuit, preferably wherein these anodes are suspended from a riser.

NON-LIMITING EXAMPLE

The invention is further elaborated by way of a presently preferred embodiment, shown in the perspective drawing of FIG. 1. FIG. 2 shows field measurements.

Reference signs in the drawing: riser (seawater transport tube) 1, caisson (casing) 2, flange 3, bracelet 4, pump 5, anode 6, centraliser 7, motor 8.

Illustrated are in FIG. 1 a casing (partly in section to reveal the components inside) of carbon steel, extending in upright position in the sea as part of a marine structure. Its bottom end is below and its top end (not visible) above sea level. Concentrically inside it extends a stainless steel seawater transport tube carrying at its bottom end as an extension a seawater liftpump of stainless steel or bronze, downward extended by the drive motor (made of stainless steel) of this pump. The liftpump and its drive motor are completely within the casing, or stated differently, both the bottom end of the liftpump and the bottom end of the drive motor are above the bottom end of the casing. The transport tube projects upward beyond the casing to extend further across the marine structure towards a seawater consuming device or its related reservoir, e.g. for cooling purposes.

The assembly of liftpump and associated drive motor is mounted to the bottom end of the transport tube by way of a flange coupling, as shown. The drive motor, liftpump and transport tube are galvanically connected through their facing contact surfaces. At a location (not shown) at a distance above the liftpump, the transport tube is galvanically connected to the casing.

Thus, there is a circumferential gap or annular space between the assembly of drive motor, liftpump and transport tube, and the casing. This gap is by nature filled with seawater entering through the bottom end of the casing and equalling sealevel, thus the drive pump, liftpump and lower part of the transport tube are completely submerged in seawater, which is also the case for the lower part of the casing.

The water intake of the liftpump is at the level of the lower end of the liftpump and debouches in the annular space within the casing. Thus, when operating, the liftpump sucks seawater from said annular spacing, which seawater is replaced by seawater from the surroundings, entering the annular space through the bottom end of the casing.

The illustrated metal structure suffers from galvanic corrosion and is for that reason protected by anodes. For maintenance purposes the assembly of transport tube and liftpump and drive motor can be extracted upwardly from the casing.

Above the mounting flange, indicating the transition from transport tube to liftpump, an assembly of anodes is shown inside the annular space. This solution for cathodic protection of the transport tube is known as such. This known assembly is associated with the transport tube. Elongated anodes of equal length co-extend and are equally distributed in circumferential direction around the transport tube, keeping a distance both to the transport tube and the casing. The longitudinal ends are at equal level. The back of these anodes, facing the transport tube, is mounted to a backing substrate. At their top and bottom and these anodes are, through their backings, connected to a relevant fixing bracelet which is secured to the transport tube. These closed loop fixing bracelets tightly fit around the transport tube. Thus these anodes are mechanically supported at both their longitudinal ends by the transport tube.

From the fixing bracelet just above the illustrated flange, the inventive assembly of anodes according to an embodiment of the invention is suspended. Said fixing bracelet is the only fixture of this inventive assembly. Thus, this inventive assembly extends floatingly or in cantilevered fashion from said fixing bracelet.

Illustrated are three axially spaced sets of anodes, making up the inventive assembly, extending from just above the liftpump with drive motor downward to end approximately level with the bottom end of the drive motor. Each set contains a number, in this case three or four, circumferentially distributed, mutually spaced anodes of equal length, co-extending such that their longitudinal ends are at equal level (registered). Different from the anodes associated with the transport tube, the anodes of the inventive assembly have their backs facing the casing such that the electrons supplied by the anodes experience more resistance to travel towards the casing compared to the liftpump, thus the electrons supply towards the liftpump is favourite.

Three spacing bracelets are shown, one in each two spaces between two axially successive anode sets and a third at the free end of the lowest set. Each spacing bracelet provides a closed loop and is sufficient rigid to individually maintain its circular shape when subjected to the loads typically prevailing at the relevant location within the annular space. The anodes are fixed with their longitudinal ends (through their backing) to these spacing bracelets.

The spacing bracelets are spaced from both the casing and the liftpump, its drive motor and the transport tube and thus locally subdivide the annular space into an inner annular space and an outer annular space. At their outward directed surface circumferentially distributed spacers are provided which bear against the inner wall of the casing while galvanic contact through these spacers between the anodes of the inventive assembly and the casing is avoided, e.g. by providing spacers made of galvanically insulating material.

Thus the inventive assembly comprising the longitudinally extending anodes and the rigid spacing bracelets between them and at the one longitudinal end, bearing against the casing wall to be maintained centred between and remote from the liftpump and the casing, provide a rigid monocoque or shell type, cantilever circumferential cover or cage for the liftpump, splitting the annular space in an inner and outer annular space. Since this cover is merely fixed at its top end, above the liftpump, to the transport tube, no modification to the liftpump is required while at the same time the upward water flow through the annular space towards the water intake of the liftpump is not obstructed since at that side, thus at the bottom, the cage has an open longitudinal end contrary to the top end which is relatively closed by the mounting elements for fixture to the transport tube.

Also illustrated is a continuous metal strip running from the fixing bracelet just above the flange all the way down to the bottom spacing bracelet. This strip is upward continued by a further metal strip bearing electronic components. This assembly of metal strips and electronic components provide the means for automatic galvanic potential control between the anodes and their associated items to be galvanically protected at changing resistance due to e.g. changing conductivity of the seawater, changing temperature, etc. The anodes are merely through these metal strips galvanically connected to the transport tube. The electronic components provide an electronic switch and possibly a rechargeable electronic accumulator, e.g. a capacitor which is preferably charged by the associated anodes.

FIG. 2 shows results from field measurements for the embodiment of FIG. 1, wherein the diameter of the casing was 1 metre, its length 22 metres and the diameter of the riser 0.5 metre (20 inch). The continuous line is according to the invention. The two broken lines show the case without sacrificial anodes or with prior art application of sacrificial anodes, respectively. Clearly, the invention provides most balanced cathodic protection across the complete length of the structure, while prior art cathodic protection is clearly absent in the area of the pump inlet position. 

1-5. (canceled)
 6. Assembly for the cathodic protection of a from galvanic corrosion suffering active component, which transports corroding medium, within a steel casing containing a corroding medium, wherein anodes of substantially equal length are suspended from only a single longitudinal end and are circumferentially distributed around the active component while co-extending next to each other and being spaced from both the casing and active component.
 7. Assembly according to claim 6, wherein remote from their suspended end the anodes are interconnected by a circumferentially extending spacing bracelet, providing a closed loop around and spaced from both the casing and active component and galvanic isolated spacers are active between the assembly of anodes, on the one hand, and the casing and active component, on the other hand, to keep the spacing bracelet, and thus the associated anodes, in a predetermined, centred position relative to the casing and active component.
 8. Assembly according to claim 7, wherein the active component is a water pump which is concentrically mounted within a circumferentially spaced carbon steel protective tube like casing located in a body of seawater, the annular gap between active component and casing is flooded with said seawater, which gap has a width at least 10 centimetre and not more then 50 centimetre and the diameter of the casing is at least 1 metre and not more then 2 metre, while the active component is galvanically more noble relative to the casing, since the active component is made from bronze or stainless steel, substantially for its complete length and the casing is substantially continuous along its complete length and the active component is an extension of another component wherein said two components are galvanically mutually connected, while the anodes are suspended from their top end and are with their top end fixed above the active component while their bottom end is at the level of the active component or its drive and the back of the anodes face the casing, which back of each anode is completely covered by a galvanically isolating coating, and each anode covers an axial length of at least 1 metre and the anodes and the component(s) they protect are mounted such that they exclusively make galvanic contact with each other through the corroding seawater and a switching galvanic circuit.
 9. Assembly according to claim 8, wherein a plurality of anodes is equally distributed across the annular gap, circumferentially and axially, while avoiding that these anodes are galvanically contacting the active component, the casing and any other component within the casing, other then through the corroding seawater, and the switching galvanic circuit and these anodes are suspended from a riser concentrically contained within the casing and providing the extension of the active component and the annular gap. 